The present invention relates generally to a pump, more particularly a steering assistance pump.
Pumps for the purposes of steering assistance are usually of the rotary vane pump type, comprising a housing in which are disposed a pressure plate, a cam ring and a rotor for defining at least one displacement region. The displacement region is divided up into a plurality of chambers by means of vanes which are carried slidably by the rotor. When the rotor is driven in rotation by a shaft, the above-mentioned chambers move between inlet and outlet ports and openings. The inlet ports communicate with a hydraulic fluid supply system while the outlet ports communicate with a hydraulic fluid discharge system, which communicate with each other by way of a flow control valve. The flow control valve includes a spool, a valve spring and a restrictor throttle or orifice at which a pressure drop is taken off and passed to the two sides of the spool. The flow control valve usually also includes a pressure relief valve which responds when a given limit pressure is exceeded and relieves a part of the controlled output flow of the pump, into an internal inlet duct or passage of the pump.
The steering assistance pump is connected by way of an external output service line to a servo steering means which normally passes on the controlled output flow to the tank. However, when the steering assembly is in a position of extreme steering lock, or angle of turn, it may happen that the servo steering means almost shuts off the controlled output flow completely, so that the pressure rises greatly and the pressure relief valve responds. A large amount of power is converted at the pressure relief valve so that there is a substantial rise in the temperature of the hydraulic fluid, especially as the hydraulic fluid is being circulated within the pump. That condition results in the pump suffering from damage and even failure, after a certain period of time.